It was a traumatic and lingering death. The adult female and young male probably fell through a fissure in a cave roof and remained alive for days or weeks with little or no food before finally meeting their end. The pair – possibly a mother and her son – were then washed by a rainstorm into an underground pool where they gradually solidified into rock.

Their unusual demise nearly 2m years ago, and the preservation of most of their fossilised skeletons, has given scientists a unique glimpse of what kind of creature they were. The researchers who have studied them in detail believe they may be direct ancestors of modern humans.

Through a combination of high resolution scans and precision measurements of the skull, pelvis, hand and foot, the authors argue that Australopithecus sediba, or the "southern ape", was an immediate ancestor of Homo erectus, the ancient form from which modern humans arose.

"What is remarkable about Australopithecus sediba is that, as a field, it is a discovery we never thought would be made: a bona fide transitional species," Berger told the Guardian.

"It is a humbling experience. These are skeletons that you realise are going to be studied by humans for as long as humans study themselves. And that gives you some pause," he added.

At least 25 other animals died alongside them, including sabre-toothed cats, hyenas, woodland antelope and at least one primitive form of zebra. Around the cave was a sub-tropical alpine forest, with mixed woodlands and forests, Berger said.

A. sediba walked upright and stood around 1.3m tall. It had a chimp-sized body, long arms similar to those of orang-utans, and was adept at climbing.

But other features appear distinctly human, Berger said. "The pelvis is shaped like a human pelvis, but longer, almost like a Neanderthal's. The hand is incredibly human-like, with short fingers and a long thumb. And then there is the brain," he added.

Researchers used a powerful x-ray scanner at the European Synchrotron Facility in Grenoble, France, to create exquisitely detailed maps of the interior of the skull of one of the individuals. The bumps and other contours revealed the imprint of a small brain, only 420 cubic centimetres in volume, but one that was apparently reorganising from a primitive structure into a more modern form.

Kristian Carlson, a colleague of Berger's who worked on the brain scans, said some areas of the organ appeared more developed than expected.

"There are areas above and behind the eyes that are expanded and they are responsible for multitasking, reasoning and long-term planning. These are changes that mirror the differences that humans exhibit from chimpanzees," Carlson said. The discovery challenges the previously held theory that our ancient ancestors grew large brains before they reorganised to resemble the modern human brain.

Further measurements of the brain, skull and hand suggest that the creature may have been intelligent enough to wield tools and even communicate non-verbally, Berger said. "They could probably smile, and that is something unique to humans that chimps cannot do, they grimace. Australopithecus sediba has the beginning of our face," he said.

Other palaeontologists have yet to be convinced that the creature was an immediate ancestor of H. erectus - and hence our own species H. sapiens. But if Berger is correct, the fossils fill a gap between Lucy, the 3.2m-year-old hominin unearthed in Ethiopia, and H. erectus, which lived from 1.8m to 1.3m years ago and likely gave rise to modern humans in Africa.

"No matter where this species is eventually put in the family tree, whether you agree with our idea that it's the best candidate ancestor of Homo erectus, or whether it is a species that mimicked the developmental processes that led to our genus, or whether it turned out to be an evolutionary dead end, these are some of the finest transitional fossils that have ever been discovered for any mammal species, and I don't say that lightly," Berger told the Guardian.

"In that view, the South African australopithecines were side-branches in human evolution, leading only to extinction. These new and detailed descriptions of the skeletons of two individuals from the Malapa site return the spotlight to South Africa as the possible location for the postulated transition from Australopithecus to Homo.

"Australopithecus sediba resembles its presumed local ancestor, Australopithecus africanus, in its ape-sized brain, ape-like body shape, and the form of the shoulders and arms. Yet despite the fact that the hands had a powerful grip, they show more human proportions, suggesting greater dexterity. And the shape of the front of the brain cavity, the face, teeth, pelvis and legs also show more human characteristics, confirming that sediba is the most human-like australopithecine yet discovered, providing valuable clues to the evolutionary changes that led to the genus Homo.

"However, it is possible that australopithecines in different parts of Africa were taking up tool-making, meat-eating and travelling longer distances overground, which could have driven the parallel evolution of human-like features," he said.